Ion sources have been used for various purposes in various fields of technology, e.g. for doping impurities into semiconductor devices, liquid crystal TFT or solar cells, for etching objects by ion beams, for cutting or polishing by ion sputtering, for depositing thin films by ion beams, for improving the quality of objects or for processing objects on some purpose.
Narrow ion beams with small diameter have also been utilized. In general, narrow ion beams have been used more frequently for measuring properties of objects than for processing or treating objects, because narrow ion beams are inconvenient to process or treat objects with high yield.
In the case of narrow ion beams, it is easy to mount a mass separation device at a point of a passage of ion beams. A mass separation device is a device which bends the paths of ion beams by magnets into curved paths with different curvatures, distinguishes ions by the difference of mass and permits the ions with predetermined mass to pass through the device. Such ion sources that produce narrow ion beams are called here zero-dimension type.
However, if the purpose of an ion source is to process objects, wide ion beams are more convenient than narrow ones, because many more objects can be processed in the same process time. The ion sources that can produce wide ion beams are now called large area type.
In the case of wide ion beams, it is difficult to separate ions by mass. Wide diameter hinders the mass separation of ion beams. Theoretically, big magnets could separate ions by mass effectively even for wide ion beams. Because of high kinetic energy and wide diameter of ion beams, mass separation has hardly been done for large area type ion sources which generate wide ion beams.
In general, ion beams have energy as large as 10 keV to 200 keV at an outlet of an ion source. In the case of large area type ion sources, the diameter of ion beams is wide. Bending these wide, strong ion beams would require a large, strong magnet with strong magnetic flux. The diameter of the poles of the magnet must be far larger than that of ion beams. Furthermore, the magnet must have an arc shape which coincides with the curvature of bent ion beams.
It is uneasy to manufacture such a huge, strong magnet. It is still difficult to mount the big magnet at the outlet of an ion source.
By these reasons, conventional large area type ion sources have rarely been equipped with mass separation device so far.
Large area type ion sources which generate wide beams are used for finishing, processing or improving objects or for doping objects with impurity. It is preferable that the ions bombarding objects are restricted to a certain kind of ions which have a definite mass. However, various kinds of ions are excited in an ion source. Without a mass separation device, undesirable ions are not eliminated. Such undesirable ions also bombard objects, which would bring about some inconvenience. Therefore, large area type ion sources also require a mass separation device.
However, conventional mass separation devices which would bend the passage of ion beams by a big magnet with a pair of wide pole pieces would be unpractical because of the huge size of the magnet. A purpose of this invention is to provide a large area type ion source with a mass separation device without increasing the size of an ion source. Another purpose of the invention is to provide a large area type ion source with a mass separation device which works under weak magnetic field and low electric field.